This invention relates generally to cryogenic fuel storage and more specifically to an LNG fueling system for internal combustion engines utilizing saturated cryogenic fuels in a system which mixes both liquid and vapor before supplying the engine.
Fuels such as liquefied natural gas (LNG) or liquid hydrogen are stored in insulated tanks in which either the liquid or vapor can be delivered to an engine on demand. Systems such as illustrated in U.S. Pat. No. 4,080,800, deliver the fluid or vapor without outside pressurization and are referred to as "saturated liquid" systems. Systems of this type supply their own pressurization by the vaporization of liquid.
Normally the liquid and vapor lines of such a fuel system are controlled by separate valves before they are joined together as they flow into the mixing valve of the engine. These valves are conventional open and close two-position valves which open in the fluid line normally when the pressure drops below a certain rate while the valve in the vapor line remains closed until the pressure in the tank exceeds a certain elevated level. Once the vapor is allowed to flow to the engine, the pressure in the tank drops quickly. The vapor flow alone is normally not adequate to provide the amount of flow at high engine demand levels which can cause supply line starvation or erratic engine performance.
Some prior art systems vent the high pressure vapor to atmosphere across a relief valve rather than utilize the vapor in the engine. A more efficient system for effective operation requires that there be means for permitting both vapor-phase flow and liquid-phase flow to meet the varying demands of the vehicle engine and keep sufficient pressure in the tank.
Storing cryogenic fuels in a saturated liquid state is most desirable since it provides a means for expulsion of the fuel at high flow rates without the need for auxiliary pressurization. A cryogenic fuel as a saturated liquid is a condition wherein the liquid is in equilibrium with the vapor. However, due to the influx of heat into the cryogenic liquid which raises its saturation pressure, it is desirable to occasionally supply vapor alone to the engine, which lowers the saturated pressure of the liquid and vapor. In saturated liquid storage systems this has previously been done by having separate on/off pressure-actuated solenoid valves wherein the vapor line valve opens at relatively high pressures and the liquid line valve opens only at low pressure levels. However, there are several drawbacks to this method of pressure control. If, for example, the valves are both opened simultaneously, a rapid pressure increase sometimes occurs due to the pressurization loop that is formed.
In U.S. Pat. No. 4,080,800 previously mentioned, a saturated liquid system is described which attempts to solve the above-mentioned problems utilizing check valves and a pressure-actuated three-way valve.